There is clearly a need to devise methods for protecting both humans and aquatic predators, such as sharks and other members of the Elasmobranchi subclass, from harmful interactions. Various methods have been used to deter sharks and other aquatic creatures from entering or remaining in aquatic areas used by humans, including chemical, mechanical, sonic or electrical means.
The underlying mechanism of electronic deterrent devices against sharks has been a subject of differing or inconsistent approaches. On one hand, it has been proposed that the current and/or voltage associated with electricity are responsible for the repellent effect of electricity against sharks and related creatures. In Holt U.S. Pat. No. 3,683,280, for example, a pulse-generating circuit is used to produce pulses applied between a pair of widely-spaced electrodes. Holt concluded that the principle of operation was based on producing a voltage drop and current flow within the area of deterrence. Other proposals, on the other hand, have concluded that it is the electric field generated by voltage applied across antennae or electrodes that is responsible for deterring sharks and other creatures. Hicks U.S. Pat. No. 3,164,772), for example, reached this conclusion based upon the observation that sharks can be repelled by electrical discharges in salt water, even though no electrical current was measurable at the effective ranges of 20 to 30 feet and no electric shock occurred to divers or other aquatic creatures. The predominant view today is that sharks and other members of the Elasmobranchi subclass are repelled by the electric field generated by current flowing through water, as opposed to the voltage potential or current itself.
Although the exact mechanism of shark sensitivity to electric fields is not fully understood, it is clear that the use of electricity in seawater offers certain advantages and disadvantages. First, electric fields are theorized to negatively affect the sensory organs in the nervous system of sharks and other Elasmobranchi—particularly sensors located in the shark's nose, as noted in Hicks and Stowell U.S. Pat. No. 4,211,980. These and other references describe how electric fields interfere with the natural timing of the shark's nervous and sensory systems causing irritation or even death. Specifically, pulse patterns of direct current (DC) in the 6-12 count-per-second (cps) range are reported to affect optimal deterrence against sharks. It is also reported that sharks and other Elasmobranchi are relatively insensitive to the direct effects of continuous DC and alternating currents applied to sea water. In contrast, humans and other sea life unrelated to sharks lack this observed sensitivity to electric fields, while exhibiting profound sensitivity to both direct and alternating current. Thus, the use of electric fields to deter sharks and related creatures offers the advantage of selectivity as compared to other physical and chemical methods.
The disadvantage of using electricity to generate electric fields in seawater is that ionic buildup at the antenna or electrodes can limit the effective range and efficiency of such devices. As explained by Stowell, the application of voltage to an antenna or electrode immersed in salt water can cause ions in the water to migrate to the pole of opposite polarity, which results in the creation of “ion gradient” (or so-called “ion shield”). This ion gradient can act as a “potential barrier” that limits the further flow of electrons to the anode, thereby causing a reduction in the electric field transmitted to the surround water. After formation of an ion gradient, prohibitively-high voltage and current levels are required to produce an effective electric field. According to Stowell, the effects of the ion shield can be mitigated by allowing sufficient delay between DC pulses to allow dissipation of the ion gradient. This so-called “off time” (pulse delay) is generally 1 to 2 orders of magnitude greater than the so-called “on time” (pulse width), which may greatly limit the effectiveness deterrent devices employing DC pulse patterns. Thus, there is a need to devise electrical deterrent devices avoiding the adverse effects of ion buildup on an electronic shark deterrent device in saltwater.
An original approach to a method and device for electronic shark deterrence is disclosed in commonly-owned U.S. patent application Ser. No. 12/238,185, filed on Sep. 25, 2008, which is incorporated herein by reference for an explanation of the general background for the improvements disclosed herein. In the original approach, an electronic device creates an output waveform for transmission from at least one pair of electrodes, wherein the output waveform comprises high voltage pulses in the range of 20 to 100 microseconds (μs) having pulse-burst durations of one-half to two-and-one-half seconds repeated at regular intervals.